On Mon, Oct 17, 2016 at 10:04 PM, Kevin Grittner <kgri...@gmail.com> wrote: > On Sun, Oct 16, 2016 at 9:26 PM, Masahiko Sawada <sawada.m...@gmail.com> > wrote: > >> When I set old_snapshot_threshold = 0 I got error at step #3, which >> means that the error is occurred without table pruning. > > The "snapshot too old" error can happen without pruning, but only > because there is no way to tell the difference between a page that > has been pruned since the snapshot was taken and a page which has > had some other kind of modification since the snapshot was taken. > > Ignoring false positives for a moment (where the page is updated by > something other than pruning), what is required for early pruning > is that the snapshot has expired (which due to "rounding" and > avoidance of locking could easily take up to a minute or two more > than the old_snapshot_threshold setting) and then there is page > pruning due to a vacuum or just HOT pruning from a page read. At > some point after that, a read which is part of returning data to > the user (e.g., not just positioning for index modification) can > see that the snapshot is too old and that the LSN for the page is > past the snapshot LSN. That is when you get the error. >> We have regression test for this feature but it sets >> old_snapshot_threshold = 0, I doubt about we can test it properly. >> Am I missing something? > > This is a hard feature to test properly, and certainly hard to test > without the test running for a long time. The zero setting is > really not intended to be used in production, but only to allow > some half-way decent testing that doesn't take extreme lengths of > time. If you add some delays of a few minutes each at key points > in a test, you should be able to get a test that works with a > setting of 1min. It is not impossible that we might need to add a > memory barrier to one or two places to get such tests to behave > consistently, but I have not been able to spot where, if anywhere, > that would be.
Thank you for explanation! I understood. When old_snapshot_threshold = 0, it skips to allocate shared memory area for the xid array and skips the some logic in order to avoid using the shared memory, so I was concerned about that a little. Regards, -- Masahiko Sawada NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
